In the past year we have continued our biologic and molecular characterization of different viral components isolated from Friend Murine erythroleukemia virus complex. Using a protocol developed in this laboratory for cloning in tissue culture Spleen focus forming virus (SFFV) free of helper virus, we have analyzed different isolates of SFFV obtained from several independent strains of Friend virus complex, including Lilly-Steeves, Mirand, and Axelrod strains of FV-P (polycythemia-inducing Friend Virus), and we have found that each of these isolated of SFFV contains xenotropic-MCF related env gene sequences. Furthermore, nonproducer cells containing each of these isolates of SFFV can be used to detect proteins coded for by SFFV. The relationship of a rapid undifferentiated leukemia induced by F-MuLV to erythroleukemia induced by SFFV has been explored.